ESPAR (Electronically Steerable Passive Array Radiator) antennas including a plurality of non-feed elements to which variable-reactance circuits are connected and a single feed element have been developed as variable-directivity antennas (for example, see Non-Patent Document 1 and Patent Documents 1–3).
Referring to FIG. 7, a known ESPAR antenna will be described.
FIG. 7(A) is a perspective view of main portions of an antenna apparatus, and FIG. 7(B) is a side view of the main portions. The antenna apparatus includes a ground conductor 1, a feed element 60 disposed at the central part of the ground conductor 1, and a plurality of non-feed elements 61a to 61f disposed around the feed element 60. Variable-reactance circuits including varactor diodes are disposed between these non-feed elements 61a to 61f and the ground. FIG. 7(B) shows the non-feed elements 61b and 61e to which variable-reactance circuits 62b and 62e are connected. A feeder circuit 30 is connected to the feed element 60.
The case in which radio waves are transmitted from the antenna apparatus, that is, the case in which power is supplied from the feeder circuit 30 to the feed element 60, will be examined. In the antenna apparatus with the above-described structure, electromagnetic coupling between the feed element 60 at the center and the peripheral non-feed elements 61a to 61f is actively employed. The radiation directivity (radiation pattern) of radio waves transmitted from the antenna apparatus is determined by the state of the electromagnetic coupling. When the reactances of the variable-reactance circuits connected to the peripheral non-feed elements 61a to 61f change, so does the state of the electromagnetic coupling. As a result, the radiation directivity of the antenna apparatus changes.
For example, as shown in FIG. 7, the feed element 60, which is a monopole antenna, is disposed at the center of the disc-shaped ground conductor 1, and, about one-quarter wavelength from the feed element 60, the non-feed elements 61a to 61f including six monopole antennas are circularly disposed at intervals of 60 degrees. Varactor diodes are used as the variable-reactance circuits. By appropriately setting voltages applied to the varactor diodes, the radiation directivity of the antenna apparatus in a horizontal plane can be controlled.
A multi-channel antenna apparatus for reducing an effect of coupling among element antennas excited at different frequencies, which is caused by the element antennas being disposed in the same aperture, is described in Patent Document 4. Non-Patent Document 1: Takashi Ohira and Kyouichi Iigusa, “Denshi-sousa Douhaki Array Antenna (Electronic Scanning Waveguide Array Antenna)”, IEICE Trans. C, Vol. J87-C, No. 1, January 2004, pp. 12–31    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-16427    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-24431    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2002-16432    Patent Document 4: Japanese Unexamined Patent Application Publication No. 9-139626
A plurality of different frequency bands may be used by devices or systems used for the same purpose. For example, the standards for wireless LANs include the IEEE 802.11a using the 5.2 GHz band and the IEEE 802.11b/g using the 2.4 GHz band. To configure an access point covering both frequency bands, a single antenna that covers these two frequency bands is necessary.
However, the ESPAR antennas described in Non-Patent Document 1 and Patent Documents 1 to 3 are used in only one frequency band and are not intended to be used in a plurality of frequency bands at the same time or at different times.
Regarding the antenna apparatus described in Patent Document 4, active directivity control, such as that performed by non-feed elements in an ESPAR antenna, cannot be performed in a plurality of frequency bands.
Conceivably, an antenna apparatus covering a plurality of frequency bands may be configured by disposing a plurality of ESPAR antennas, each operating as an ESPAR antenna in one frequency band, on a single ground conductor. However, the directivity of an ESPAR antenna changes due to electromagnetic coupling between a feed element (radiating element or radiator) and non-feed elements (waveguide elements or directors). When feed elements and non-feed elements operating in a plurality of frequency bands are simply disposed on the same ground conductor, the radiation directivity in a desired frequency band is negatively affected by coupling between a feed element and non-feed elements in an undesired frequency band. As a result, the desired radiation directivity cannot be achieved.
Problems similar to those above occur when the radiation directivity with respect to wireless frequency signals in different frequency bands is controlled or when the feeding position in the structure of a diversity antenna is changed.